Aluminum metal is one of the most widely used materials in many fields of industry because of its excellent characteristics, such as good workability and light weight. Aluminum may be used in its pure form (approximately 99% elemental aluminum) as extracted from natural mineral sources (such as bauxite), but more often is used as an alloy with one or more other earth metals, such as copper, silicon, magnesium, manganese, nickel or zinc. An alloy is a uniform mixture or combination of metals or metallic compounds, but is not itself a compound. The mixture can be a solid solution, can contain mutually insoluble metals, and can form a crystal lattice, and the resulting alloy will usually have physical characteristics that differ from those of its component metals.
Because of insufficient wear resistance and corrosiveness (due to softness and oxidizability) aluminum and aluminum alloys are often subjected to a variety of surface treatments. Typical examples of surface treatments for aluminum metal are anodic oxidation, vacuum deposition, electroplating, surface coating, cladding and spraying.
Anodic oxidation of aluminum is an electrochemical process in which aluminum is anodically oxidized in an aqueous electrolyte to form an aluminum oxide (alumina, or Al.sub.2 O.sub.3) on its surface. In vacuum deposition, a metal is vacuum-deposited on the surface of the aluminum. Electroplating provides that the aluminum surface is first zinc-coated (a zincate treatment) or nickel-coated and then electroplated with a predetermined metal. Electroless plating and synthetic resin coatingtreatments may be used, in which an aluminum surface is coated with synthetic resin. Cladding is a method wherein an aluminum surface is cladded with thin copper sheeting by an applied organic adhesive. There is also a process in which the aluminum surface is sprayed with another metal or ceramic.
Anodic oxidation is of particular interest in the present invention. When an aluminum substrate is made the anode in an electrolytic circuit, negatively charged anions are moved toward the anode and discharged. In an aqueous electrolytic solution, some of the anions are oxygen, which forms a chemical bond with the aluminum anode to form aluminum oxide. The oxidation reaction is affected by various treatment conditions, such as the properties of the electrolyte, the temperature, current, voltage, treatment time, etc. In most applications, anodic oxidation requires some form of pretreatment of the substrate, such as mechanical treatment (e.g., sand-blasting), chemical treatment (e.g., dipping in caustic soda) or electrical treatment (e.g. electrolytic degreasing).
Additional factual background concerning aluminum, its alloys and anodic oxidation can be found in Van Nostrand's Scientific Encyclopedia, Van Nostrand's Chemical Dictionary, and in the Kirk-Othmer Concise Encyclopedia of Chemical Terminology, at entries for aluminum, anodic oxidation, alloy, metal treatments, and welding.
The known treatments for aluminum and its alloys have many disadvantages. For example, while anodically oxidized aluminum is used in quite a few fields, its hardness does not necessarily meet the requirements of some applications, and its wear resistance may be insufficient. For example, micropores in the oxidized anodic coating may result in poor resistance to corrosion. The vacuum deposition treatment is expensive and impracticable for bulky materials and mass-production. Electroplating is widely used, but due to the high ionization of the aluminum itself, it is difficult to plate copper directly onto aluminum. In general, therefore, zinc and nickel are first electroplated on aluminum as undercoats and copper or chrome is thereafter plated as another layer. This method involves extra steps, such as troublesome pretreatments (e.g. degreasing), and suffers from various disadvantages. For example, electroplated metals are insufficiently stable as adhesives on an aluminum substrate.
Cladded materials are complex and use adhesives in most cases, resulting in heat resistance problems. During a heat cycle (alternating increasing and decreasing temperature), differences in the expansion coefficient between aluminum and the coating metal cause a flaking phenomenon. Similarly, metal spraying treatment (for spray coatings) entails severe problems in adhesive strength; for example, a sprayed ceramic cannot endure any secondary working or mechanical deformation, such as bending.
Background material concerning the general behavior of ceramic materials can be found in Frankel, J., Z: phys 35 652 (1926) and C. Wagner and W. Schotzky, Z: phys B 11 (1931).
Ceramic materials containing copper and alumina are known as Spinel ceramics, and can be represented by the formula Cu.sub.x Al.sub.2 O.sub.3, where x is an integer. These materials are not well understood, and have not been known to be useful as surface treatments for aluminum substrates. Successful and integrally formed complex aluminum-copper-ceramic materials having a desirable laminar structure and a homogeneous surface layer were not known prior to this invention, nor were the benefits of the structure and method of the invention heretofore predicted.